I haven't been to siggraph, but for $1,175 ( http://www.siggraph.org/s2010/for_attendees/registration ) there's surely a potential for a contract between you and the ACM via your conference registration.

Also, copyright holders don't need to enter into a contract to assert their rights. You don't enter into a contract when you watch TV but you still don't get the right copy and distribute shows (or sporting events) that you watch.